Apparatus and method for testing battery condition

ABSTRACT

In circuits and systems embodying the invention the condition of a battery may be tested and monitored while the battery is supplying a load current to a load and without interrupting that operation. A selectively enabled switch causes a test current to selectively flow through the battery and a test resistor of known value. The voltage across the test resistor is sensed to determine the value of the test current. The voltage across the battery is measured under two conditions, with the test current and without the test current flowing through it. These measurements and knowing the value of the test current enable the internal resistance of the battery under test to be calculated and monitored.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from provisional patentapplication Serial No. 60/271,472 for Battery Life Monitoring Devicefiled Feb. 26, 2001 by Gary R. Hoffman, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a method and apparatus for monitoringselected electrical characteristics of a battery used to power anelectrical system.

[0003] Batteries are used in many critical applications. For example,stationary storage batteries may be used to power equipment andcomponents in electrical power generating and/or distributionsubstations. When used, the batteries must supply the needed current andpower to operate circuits used to enable the orderly shut down orde-energization, (or even the re-energization and turn-on) of criticalcontrol circuits within the substation. It is therefore important thatthe battery be in a condition to supply the power needed to operate thesystem reliably. However, with the battery connected in circuit, it isdifficult to determine the condition of the battery and/or whether it isat, or close to, a failure point. In the case of unmanned sites, such asa remote substation, skilled personnel may be routinely dispatched tothe remote locations to check out the condition of these batteries.However, this is expensive and inefficient.

[0004] These batteries are “heavy duty” batteries and may be, forexample, of the lead acid type, nickel cadmium type and/or any othersuitable type. As a storage battery ages as a function of usage, careand temperature, electrochemical changes take place within the batteryand also at the connections to the battery. The electrochemical changesmay give rise to an increase in the resistance of the battery that mayinhibit the battery from delivering the necessary current.Alternatively, the electrochemical changes may give rise to an unwanteddecrease (e.g., a short) in the resistance of the battery. It istherefore desirable and/or necessary to continuously monitor theresistance of the battery and/or the ability of the battery to providethe required power output to a load.

[0005] Some known techniques to measure battery resistance rely on theinjection of AC currents into the battery for measuring both itsabsolute impedance and changes in the impedance of the battery. However,the circuitry needed to accurately measure the change in the impedanceto a required degree of resolution renders these techniquesprohibitively expensive in most applications.

[0006] These problems are overcome in circuits and systems embodying theinvention.

SUMMARY OF THE INVENTION

[0007] In circuits and systems embodying the invention the condition ofa battery, supplying a load current to a load, may be tested andmonitored while the battery is connected to the load and withoutinterrupting that operation. A selectively enabled switch causes a testcurrent to selectively flow through the battery and a test resistor ofknown value. The voltage across the test resistor is sensed to determinethe value of the test current. The voltage across the battery ismeasured under two conditions, with the test current and without thetest current flowing through it. Making and processing thesemeasurements and determining the value of the test current enable theinternal resistance of the battery to be calculated at any point in timeand to be monitored over time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the accompanying drawings like reference characters denotelike components; and

[0009]FIG. 1 is a simplified schematic diagram of a circuit embodyingthe invention;

[0010]FIG. 1A is a block diagram of typical load components to bepowered by a battery;

[0011]FIG. 2 is a simplified schematic diagram of another circuitembodying the invention;

[0012]FIGS. 3 and 3A are other schematic diagrams of circuits embodyingthe invention;

[0013]FIG. 4 is a block diagram of a system embodying the invention; and

[0014]FIG. 5 is another diagram of a circuit embodying the inventionusing a single battery.

DETAILED DESCRIPTION OF THE INVENTION

[0015]FIG. 1 is a simplified schematic diagram of a circuit embodyingthe invention. A Battery 10 which is intended to, supply the power to aload RL is connected between power terminals 12 and 14. Battery 10,whose condition is to be tested and monitored, represents the DC powersupply used to power the load (e.g., RL) in the system (e.g., anelectric power generating substation). Typically, battery 10 may be inthe range of 12 to 250 volts. However, it should be understood that anybattery of virtually any voltage value may be used in circuits embodyingthe invention. In FIG. 1, terminal 12 denotes the positive terminal ofthe battery supply system and terminal 14 represents the battery groundterminal. A load, RL, connected between terminal 12 and 14, is intendedto represent all the diverse loads in the system powered by the battery10. For ease of illustration RL is shown as a resistor; however RL, asshown in FIG. 1A may be a compound load with inductive and capacitivecomponents, or an active load. By way of example, FIG. 1A shows a load(RL) which includes breaker coils, protective relays, a transducer, anda remote terminal unit (RTU),160, for communicating with acentral/dispatch station. However, as already noted, any suitable loaddevice may be included as part of the load to be powered by the battery.The battery 10 and the load RL form one loop in which the load current,IL, flows, with terminal 14 defining the ground return (load ground)for all the components powered by battery 10.

[0016] Referring to FIG. 1, a first operational amplifier A1 isconnected across RL and is designed to produce a voltage V1 indicativeof the voltage VL developed across the load RL in parallel with thebattery (10). Since it is necessary for battery 10 to be in a conditionto supply the power to operate the load circuits connected across thebattery, additional monitoring circuitry is coupled to battery 10 totest it and to ensure its fitness as further described below.

[0017] The battery testing circuitry includes a second loop fordetermining the condition of battery 10. The second loop includescircuitry for selectively injecting a DC test current into (or out of)battery 10. In FIG. 1, the second loop includes a current generator 20for producing the test current (I1) which is connected via a switch S1between terminal 12 and a terminal 15. It is noted that currentgenerator 20 may be a sinking or sourcing current source (i.e., it maysupply a current in the same direction as iL or in an opposite directionto iL). An auxiliary (“compliant”) power supply 22, which may be abattery, providing a voltage denoted as “V_(H)”, is shown connected atits negative terminal to terminal 15 and at its positive terminal to aterminal 17, which is defined as the internal ground return of thecircuit. Note that power supply 22 may be a battery independent ofsource 10, or even an independent dc-to-dc converter operating frombattery 10. Note that in the equivalent circuit of FIG. 1, currentsource 20 may include battery 22. However battery 22 is explicitlyshown, in addition to current source 20, to indicate that an extrabattery may be used to practice the invention. A series resistor Rs isconnected between internal ground terminal 17 and terminal 14 (loadground). Switch S1 is a selectively enabled switch whose turn-on andturn-off may be controlled by a microprocessor 40. When switch S1 isclosed, a loop (or test) current I1 flows from internal ground 17through resistor Rs through battery 10, through the current source 20and then through battery 22 back to internal ground terminal 17.

[0018] The test current I1 through Rs causes a voltage Vs to bedeveloped across resistor Rs. The voltage across Rs is sensed by anoperational amplifier A2 which produces a voltage V2 corresponding toVs. The outputs (V1 and V2) of amplifiers A1 and A2 are fed to an analogto digital converter (ADC) 30 which converts the analog values ofvoltages V1 and V2 to digital values, with ADC 30 then supplyingcorresponding signals to a microprocessor 40 for processing.

[0019] Rs may be selected to have a relatively well defined ohmic valueand its variations with temperature may also be relatively well defined.Thus, when the switch S1 is closed and a current flows through Rsproducing a voltage V2, the value of the test current I1 can beascertained to a great degree of accuracy, since I1=V2/Rs. For ease ofexplanation, and in the discussion to follow, it will be assumed that V1is equal to VL and V2 is equal to Vs. Note that current source 20 can bemade to be a relatively “stiff” current source whose current amplitudeis relatively well defined, whereby measuring V2 indicates the value ofRs as well as the value of the test current.

[0020] In the operation of the system, the value of the voltage acrossRL, which is also equal to the battery 10 voltage, can be measured withS1 open and the information supplied to, and stored in a microprocessor(e.g., 40 in FIGS. 2-5) or other suitable processing device. Then, theswitch S1 is closed and the voltage across the battery is again measuredwith the infusion of a test current I1. This value of voltage is alsosupplied to and stored in the processor. With S1 closed, the voltageacross battery 10 is now a function of the current I1 plus the currentIL flowing through the battery. The change (increase or decrease) involtage across the battery 10 with S1 closed yields the value of thebattery resistance (Rbattery), as set forth in eq. 1 below:

Rbattery=[V 1(with S 1 closed)−V 1(with S 1 open)]/[I 1];  eq. 1

[0021] where I1=V2/Rs

[0022] The values of battery voltage (V1) with and without the testcurrent injected into (or out of) of the battery and the value of V2 canbe periodically (or selectively or programmably) measured and fed to themicroprocessor 40 to enable the dynamic value of Rbattery to beperiodically or selectively calculated. The microprocessor 40 isprogrammed to perform all the calculations and is also programmed tosense whether any changes in Rbattery exceed predetermined boundaryvalues. That is, V1 and V2 are sensed and I1 is determined, since Rs isknown. Then, the value of Rbattery is calculated using equation 1.Following which, the calculated values of Rbattery may be compared to apredetermined and/or known range of permissible or acceptable values ofRbattery. The predetermined range of permissible values of Rbattery maybe prestored or preprogrammed in a memory (not shown) in processor 40 orin a memory such as EEROM 403 shown in FIG. 4. If any of the permissibleboundary values for Rbattery are exceeded, the microprocessor generatesa warning and an alarm may be sounded (see 409 in FIG. 4). Determiningthe value of Rbattery is central to the invention because its value isindicative of the current (and power) the battery can deliver. It shouldbe noted that in accordance with the invention the value of Rbattery maybe calculated and stored and/or plotted over time so that the values ofRbattery may be calculated at any time and the changes in Rbattery overtime may also be monitored, noted and recorded. This enables thedetermination of the amount of changes in Rbattery and the direction ofthe change and rate of change in Rbattery over time.

[0023] It is significant that, in accordance with the invention, thebattery resistance can be measured using a relatively simple DC circuitand that switch S1 can be opened and closed without substantiallyaffecting the operation of the battery 10 feeding the load RL, and hencethe operation of the load. By superimposing a test current (in phaseand/or out of phase) on the load current being supplied by a battery 10and by using two different current carrying loops, one for the loadcurrent and one for the test current, the condition (e.g., the internalbattery resistance) of the battery 10 can be determined. Note also thatdifferent ground returns may be used for the battery supplied loadsystem and for the test current loop whereby the load is essentiallyisolated from the test current generating network.

[0024] The microprocessor 40 may be programmed to determine the rate ofchange of the battery resistance (e.g., is it increasing or decreasingover time and to compare the sensed and calculated values versushistorical resistance data. The microprocessor 40 may be preprogrammedto include information regarding permissible changes in batteryresistance as a function of the load (e.g., current drawn) imposed onthe battery, and/or as a function of temperature and time. Themicroprocessor may then compare the rate of change in resistance againstpreprogrammed set points. Should the value of Rbattery and/or the rateof change in battery resistance be equal to or greater than certainpreprogrammed values the microprocessor can generate an alarm signal.

[0025] Referring to FIG. 2, the “main” battery 10, whose voltage isidentified as VB, is connected between terminals 14 and 12, and acompliance power supply 22, having a voltage VH, is shown as a battery22 having its negative terminal connected to the positive terminal ofbattery 10, at terminal 12 and its positive terminal connected toterminal 15. The switch S1 and the current source 20, shown in FIG. 1,are implemented in the circuit of FIG. 2 by a circuit which includesbipolar transistors T1 and T2, diode D1 and a resistor R2. Transistor T2is shown as an NPN bipolar transistor with its emitter connected toterminal 14 and its base coupled to microprocessor 40 which selectivelysupplies turn-on and turn-off signals to T2. The collector of T2 isconnected via a resistor R2 to a node 201 to which is connected the baseof a PNP transistor T1 and the cathode of a diode D1. The emitter of T2and the anode of D1 are shown connected to terminal 15. The collector ofT1 is connected to node 203 and a resistor Rs is connected between node203 and terminal 14. When transistor T2 is turned on it pulls a currenti2 out of node 201 causing transistor T1 to turn on and to supply acurrent I1 into resistor Rs. The current (I) drawn out of the battery 22and battery 10 is equal to the sum of I1 and i2. In practice i2 may bemade much smaller than I1 whereby the current I1 may be assumed to beequal to I. As is the case of the circuit shown in FIG. 1, the voltagedeveloped across Rs is applied to an operational amplifier A2 whoseoutput is applied to an ADC 30 b whose output, in turn, is applied tomicroprocessor 40. Concurrently, the voltage across the battery 10 isapplied to an operational amplifier A1 whose output is applied to an ADC30 a whose output in turn is applied to microprocessor 40, whichfunctions as discussed above. In FIG. 2, the operating potential toamplifiers A1, A2 and ADC 30 a and ADC 30 b is the voltage developedbetween terminal 15 (the most positive potential) and terminal 14(functioning as system ground. Note that in FIGS. 1 and 2 the current I(or I1) drawn by the auxiliary loop is in a direction to add to the loadcurrent iL drawn from battery 10. Thus currents I and iL flow in thesame direction through the battery 10.

[0026] Note also that in FIGS. 1 and 2 the auxiliary power source 22 maybe a battery whose value is just a few volts. For example, if thebattery 10 voltage is 12 volts, the value of VH could be any voltagegreater than approximately 1 volt. A clock circuit 210 is shownconnected to processor 40 to supply timing and clocking signals to theprocessor 40. Similarly, a clock signal 310 is shown connected tomicroprocessor 40 in FIG. 3 and a clock circuit 310 a is shown connectedto processor 40 in FIG. 3A.

[0027] Referring to FIG. 3, as before, the load powering battery 10connected between terminals 14 and 12 supplies the load current iL tothe load RL. The battery 10 is tested by injecting a test current I1,supplied from a compliance battery 22 a, into battery 10. The current I1is injected into the battery 10 in a direction opposite to the currentiL drawn from battery 10 by the load RL. In FIG. 3, the “compliant powersupply” 22 a is shown with its most negative terminal connected toterminal 14 and its most positive terminal connected to terminal 15. InFIG. 3 transistors T1 and T2 function in a similar manner to thatdescribed for FIG. 2. However, in FIG. 3 the collector of T1 isconnected to node 303 and resistor Rs is connected between node 303 andterminal 12 to which is connected the positive terminal of battery 10.When transistor T1 is turned on a current I1 is passed through Rs and isinjected into battery 10. In FIG. 3, the voltage VH of source 22 a mustexceed the voltage of battery 10 plus the voltage drop across Rs (i.e.,Vs); where Vs or V2 is equal to I1×Rs.

[0028] The circuit of FIG. 3 may be modified as shown in FIG. 3A. In theembodiment of FIG. 3A, the battery 10 is connected between terminals 12and 14 and the battery 22 is connected between terminal 17 and 15. Thecircuitry for supplying a test current includes a resistor R1A connectedbetween terminals 15 and 301. The emitter to collector path of atransistor T1A is connected between terminals 301 and 12 and a resistorRs is connected between terminals 14 and 17. Transistor T1A is turned onand off by means of a transistor T2A having its conduction pathconnected in series with a resistor R2 between terminals 17 and 201. Abiasing resistor R1 is connected between terminals 201 and 15.Transistor T1A is shown to be a PNP bipolar transistor; but transistorT1A could be any other type of suitable transistor such as a metal oxidesemiconductor (MOS) transistor. Likewise, transistor T2A is shown to bean N-channel MOS field effect transistor (MOSFET); however as indicatedin the other figures this transistor may be a bipolar transistor or anyother suitable switching device.

[0029] In the operation of the circuit of FIG. 3A, whenever themicroprocessor 40 supplies a positive going turn-on signal to the gateof transistor T2A, T2A is turned-on and pulls current out of node 201turning on transistor T1A which then supplies a test current I1 into thebattery 10 and via Rs to terminal 17. In this circuit the value of thebattery 10 voltage is measured by means of amplifier A1 for two signalconditions: (a) with I1 flowing; and (b) without I1 flowing.Concurrently, the value of the voltage across Rs is measured by means ofamplifier A2 to determine the value of the test current I1 injected intobattery 10. The outputs of A1 and A2 are supplied to processor 40 whichis set up (programmed) to calculate the value of Rbattery. The value ofRbattery may then be compared to acceptable values of Rbattery stored inmemory.

[0030]FIG. 4 shows that the battery 10 under test may be selectivelyconnected to the load, RL, via a switch S3 to form the load currentsupplying loop. The “test” current injecting loop includes the“compliant” power supply 22 connected in series with current source 20and via a switch S1 to terminal 12 of battery 10. The series resistor Rsis connected via a switch S2 to terminal 14 of battery 10. The turn-onand turn-off of these switches (S1, S2) is controlled by themicroprocessor 40 which determines if and when these switches areturnedon and off. In FIG. 4, the voltage across Rs is sensed by a bufferamplifier 401 (which corresponds functionally to amp A2 in FIGS. 1-3)whose output is connected to a multiplexer (MUX) 29 whose output(s) is,or are, coupled to A/D converter (ADC) 30. The microprocessor may alsobe programmed to control the scaling of amplifier A1 whose output is fedto MUX 29 whose output is applied to ADC 30 Scaling of the input signalsto the amplifiers may be needed to ensure that the amplifiers canrespond to the full range of the input signals. Note that the output ofADC 30 is fed to microprocessor 40 for processing, as discussed above.FIG. 4 shows an electrically erasable memory (EEROM) 403 for storinginformation including information pertaining to selected characteristicsof battery 10 and its resistance. Note that any suitable RAM or ROMcould also be used instead of EEROM 403.

[0031] As discussed above, in the operation of the circuit, values (V1)of battery voltage 10 and the voltage (V2) across Rs are measured with:(a) I1 flowing for which switches S1, S2 and S3 are closed; and (b)without I1 flowing for which 33 is closed but S1 and/or S2 are open. Themeasurements are fed to the processor 40 which then calculates values ofRbattery. The information stored in the memory may then be compared tothe measured and calculated values of Rbattery and then may be processedby microprocessor 40. The different values of Rbattery, as Rbatteryvaries over time, may be stored in memory (e.g., EEROM 403 and/or anyother memory located in processor 40) to enable the continuousmonitoring of the changes in Rbattery over time and the rate of changeof Rbattery.

[0032]FIG. 4 also shows that the microprocessor 40 may activate a liquidcrystal display (LCD) 405 which may be located within a substation or ata remote site. Likewise, the processor may generate signals transmittedvia a transceiver 407 to an RTU or to any other monitoring devicelocated within the same substation as the battery under test or to acentral or manned station. Thus, values of Rbattery and associatedchanges, may be monitored at a local substation or be transmitted to aremote (e.g., central) power station. In addition, the processor 40 maygenerate signals for activating an alarm 409. Furthermore, processor 40may be responsive to external switches 411 located on a control panel(not shown).

[0033] The battery 10 and the voltage and resistance sensing circuitryare highly suitable for use in an electrical power generation substationand or in a substation for distributing power. However, it should beunderstood that any “main” battery, wherever located and however used,may be monitored in accordance with the invention. A current may beinjected into or drawn out of the battery under test to determine thevalue of the battery resistance.

[0034] Referring to FIG. 5, the battery 10 connected between powerterminals 12 and 14 supplies the load current IL to a load RL alsoconnected between terminals 12 and 14. The battery voltage is sensed bymeans of amplifier A1 which then supplies a corresponding signal to ADC30 a which in turn supplies a corresponding signal to the processor 40.In FIG. 5 there is shown a divider network comprised of resistors RA andRB connected in series across battery 10 and the input to amplifier A1is derived from a point along the divider network. RA and RB may beequal to each other or may be selected to produce any suitable divider(scaling) ratio for the input to amplifier A1. In FIG. 5 where a singlebattery is used the input to amplifier A1 is scaled down to enable A1 tosense the full range of battery 10 voltage changes. Thus, the value ofthe battery voltage for any given load condition may be determined. Asdiscussed above, a test current (e.g., I1) may be selectively (orperiodically) drawn from the battery. In the circuit of FIG. 5, theadditional test current is drawn by means of switching circuitry whichincludes a transistor T2 which is turned on and off by processor 40supplying a turn-on or turn off signal to the base of T2. When T2 isturned on, it pulls a current (i1 a) out of the base of transistor T1which then supplies a current i1 b into a node 502. A ground returnresistor R31 is connected between node 502 and ground terminal 14. Anemitter follower transistor T3 is connected at its base to node 502 atits collector to terminal 12 and at its emitter to one end of a resistorRs. The other end of resistor Rs is connected to terminal 14. ResistorRs may be selected to have a predetermined known value whosecharacteristics as a function of temperature (and time) are known. Whena current flows into node 502 transistor T3 multiplies the current andconducts a current identified as I1 and causes that current to flowthrough Rs to ground terminal 14. The voltage developed across Rs issensed by amplifier A2 which then supplies a corresponding voltage toADC 30 b which then supplies a corresponding signal to processor 40.Since the value of Rs is known the value of I1 may be determined sinceI1 is approximately equal to [V2]/Rs. Since the current I1 is made muchlarger than i1 a and i1 b, it may be assumed that when the batterycondition testing switch is closed (i.e., T1, T2, and T3 are enabled)the test current is essentially equal to I1. The battery 10 voltage ismeasured for the condition when the testing switch is closed and I1flows and for the condition when the switch is open and I1 does notflow. This enables the microprocessor 40 to calculate the internalbattery resistance, Rbattery, as discussed above. The calculation forRbattery can be made based on the measurement of battery voltage and thevoltage across Rs. The measurements and values obtained can be comparedto pre-established acceptable values previously stored and programmed inmemory in processor 40 (or in another part of the sytem). An alert maybe given whenever Rbattery falls outside an acceptable range.

[0035] Thus, the combination of transistors T1, T2 and T3 function as aswitch and a current source supplying a current into resistor Rs. Asabove, transistors T1, T2 and T3 are shown as bipolar transistors butsuitable MOSFETs may be substituted.

[0036] In accordance with the invention, the value of Rbattery may bemonitored as it varies over time by periodically and/or programmablymaking the measurements described above. The calculated values ofRbattery may be continuously compared versus stored values inmicroprocessor 40 or they may be transmitted via transceiver (see FIG.4) to a central tracking device. Then, whenever, a battery 10 beingmonitored shows signs of approaching failure or unacceptable weakening,steps can be taken to replace the battery (or possibly recharge it) toensure continuous safer operation of the system. Thus, the battery undertest may be located in an unmanned substation and information pertainingto the battery may be transmitted to a manned station to dispatchsomeone to repair/replace any defective battery.

What is claimed is:
 1. In a system in which a load is connected across first and second power terminals of a battery, a circuit for sensing the condition of the battery comprising: a current source generating a first current; a resistor; a selectively enabled current switch, means connecting the current source in series with the resistor, the selectively enabled current switch, and the battery for selectively passing the first current through the resistor and the battery; means for sensing the voltage across the resistor; and means for sensing the voltage across the battery for the condition when the selectively enabled switch is open and for the condition when the selectively enabled switch is closed.
 2. In the system as claimed in claim 1, wherein the voltage is sensed across the resistor to determine the amplitude of the first current.
 3. In the system as claimed in claim 2, wherein the values of the voltages sensed across the battery when the selectively enabled switch is closed and then opened and the value of the voltage sensed across the resistor are for determining the value of the resistance of the battery.
 4. In the system as claimed in claim 1, wherein the voltage sensed across the resistor is coupled to microprocessor circuitry and wherein the voltage sensed across the battery is also coupled to the microprocessor circuitry and wherein the microprocessor circuitry is programmed to determine the value of the battery resistance.
 5. In the system as claimed in claim 1, wherein the battery is a first battery for supplying power to the load, with a load current from the first battery flowing through the load within a first loop; and wherein the current source generating a first current includes a second power source for supplying said first current and wherein said first current flows through a second loop which includes the first battery and said second power source.
 6. In the system as claimed in claim 5, wherein the first battery and the second power source are connected in series adding
 7. In the system as claimed in claim 5, wherein the first battery and the second power source are connected in series opposing.
 8. In the system as claimed in claim 5, wherein the first battery, the second power source, the current source, the selectively enable switch and the resistor are connected in series defining said second conductive loop for selectively passing said first current.
 9. In the system as claimed in claim 8, wherein the first battery and the second power source are poled to conduct current in the same direction.
 10. In the system as claimed in claim 8, wherein the first battery and the second power source are poled to conduct current in opposite directions.
 11. In the system as claimed in claim 5, wherein the second power source is connected between the first power terminal and a third terminal, and wherein the resistor and the switch are connected in series between the third terminal and the second power terminal.
 12. A system comprising: a battery with first and second power terminals; a load connected across said first and second power terminals with the battery supplying the power to the load, a circuit for sensing the internal resistance of the battery comprising: a current source for producing a first current; a resistor; a selectively enabled current switch, means connecting the current source in series with the resistor, the selectively enabled current switch, and the battery for selectively passing the first current source current through the resistor and the battery; means for sensing the voltage across the resistor; and means for sensing the voltage across the battery for the condition when the selectively enabled current switch is open and for the condition when the selectively enabled current switch is closed.
 13. In a system as claimed in claim 12 wherein the voltage sensed across the resistor is supplied to a microprocessor and wherein the voltages sensed across the battery are supplied to the microprocessor; and wherein the microprocessor is programmed to determine the value of the internal resistance of the battery on the basis of the sensed voltages.
 14. In a system in which a load is connected across first and second power terminals of a battery whereby the battery supplies the load current to the load, a circuit for sensing the condition of the battery comprising: a resistor of known value; a selectively enabled switch for selectively causing a test current to flow through the resistor and the battery, means for sensing the voltage across the resistor; and means for sensing the voltage across the battery for the condition when the selectively enabled switch is open and for the condition when the selectively enabled switch is closed.
 15. In the system as claimed in claim 14 wherein the voltage is sensed across the resistor to determine the amplitude of the test current; and wherein the voltage is sensed across the battery for the condition when the selectively enabled switch is open and for the condition when the selectively enabled switch is closed is for determining the value of the internal resistance of the battery.
 16. In the system as claimed in claim 15, wherein the voltage sensed across the resistor and the voltage sensed across the battery are supplied to an analog-to-digital converter (ADC) and wherein the ADC produces output signals corresponding to the voltages sensed, and wherein these output signals from the ADC are supplied to a microprocessor for determining the value of the internal resistance of the battery.
 17. In the system as claimed in claim 15 wherein the battery voltage is sensed by means of a voltage divider connected across the battery.
 18. A method for monitoring the condition of a battery, having first and second power terminals across which a load is connected and to which the battery supplies a load current, comprising the steps of: selectively causing a resistor of known value to be coupled to the battery for selectively causing a test current to flow through the resistor and the battery, in addition to the load current; sensing the voltage across the resistor; sensing the voltage across the battery for the condition when the load current and test current flow through the battery and for the condition when only the load current flows through the battery; and calculating the value of the battery resistance.
 19. The method for monitoring the condition of the battery as claimed in claim 18 further including memory means for storing acceptable battery parameters; and wherein the calculated values of battery resistance are compared against the stored acceptable parameters.
 20. The method as claimed in claim 18 wherein the test current is selectively and continuously applied and wherein the voltage across the battery is selectively and continuously sensed and the battery resistance is selectively and continuously calculated and the calculated values are continuously compared to stored parameters to ensure that the calculated values lie within an acceptable range.
 21. The method as claimed in claim 18 wherein the step of calculating the value of the battery resistance includes: (a) the step of using the voltage sensed across the resistor to determine the amplitude of the test current; and (b) the step of dividing the difference in the voltage sensed across the battery for the condition when the load current and test current flow through the battery and for the condition when only the load current flows through the battery by the test current to obtain the value of the battery resistance.
 22. The method as claimed in claim 18 wherein calculating the value of the battery resistance includes the step of supplying the voltage sensed across the resistor and the voltage sensed across the battery to a microprocessor. 